There are many types of power supply systems that convert fuel to electricity, for example, coal-fired power plants, portable home generators driven by gasoline combustion engines, automobile alternators, and fuel cells. In these power supply systems, it is important to know the amount of fuel used or the amount remaining during operation.
Fuel cells, for example, energize an increasing number of applications due to their improving efficiency and portability. Miniaturized fuel cells are often used in applications once reserved exclusively for batteries. Advances such as solid oxide electrolytes and highly specialized electrodes have allowed fuel cells to become plausible energy sources in both miniature and large-scale applications.
Fuel cells have the advantage of being able to perform non-stop power production, at least theoretically, given an endless supply of fuel. Batteries, even rechargeable batteries, eventually lose their charge and must be refreshed. Recharging a battery costs not only power, which is usually merely stored in the battery, but also time, as few batteries can be recharged instantaneously.
Gauges to indicate the remaining power in a battery are known, for example, in automotive applications that often include a voltage meter that measures, at least indirectly, whether a car battery is remaining operational. Dry cells for flashlights and home electronics also commonly include a built-in freshness indicator that approximates the amount of remaining power by displaying a color code or a bar graph of a voltage level.
In contrast to batteries, fuel consuming power supply systems connected to a limited supply of fuel, e.g., a tank, typically have no appreciable decrease in output voltage to indicate the amount of fuel used or left in the tank. For example, a fuel cell's voltage is fairly constant until the very end of the fuel supply. Thus, known methods for indicating fuel supply in these fuel consuming systems are typically mechanical. Tanks of liquid and gaseous (or liquefied gas) fuels typically rely on mechanical liquid level and gas pressure sensors, often combined with tank position sensors to compensate for tank tilting. Float-based fuel level sensors, for example, are not very accurate, are sensitive to tank position or “attitude,” and are relatively expensive. Magnetic induction fuel level sensors are inaccurate and also somewhat sensitive to tank attitude. Weight-based sensing of remaining fuel in a supply using a “scale” is expensive and bulky.
These mechanical methods work for relatively large applications, such as automotive fuel tanks although the mechanical sensing parts add cost to the tanks. For a miniature power supplies, e.g., tiny fuel cells, the mechanical sensing means become more cumbersome. For an electronic chip-sized fuel cell, it may not be practical to include a pressure gauge or level sensor.